High porosity metal biporous foam

ABSTRACT

An environmentally friendly process for producing metal biporous foam by using filamentary metal powders such as nickel or copper. The filamentary metal powders are initially wet when combined with a suitable foam former such as methylcellulose. Because the filamentary metal powder is wet, it does not extract water from the foam structure thereby ensuring a highly porous metal foam having both high macroporosity and microporosity.

TECHNICAL FIELD

The present invention relates to porous foams in general and to porous metal foams in particular.

BACKGROUND OF THE INVENTION

Porous metal foams are used in many industrial and consumer applications. Examples include filters, strong and lightweight supports, internal combustion engine exhaust collectors, pollution controls, fuel cells, catalysts, cushioning and absorbing material, electrodes for primary and secondary batteries, etc. Demand for finer porosity, greater surface area, varying metal content and other physical and chemical parameters is driving increased research and development of improved porous metal foams and methods for producing them.

For energy applications, such as alkaline cells, nickel-metal hydride batteries, lithium ion cells and fuel cells, porous metal foams act as electrodes. Typically pasted and activated with the appropriate materials, the foams are both electrodes and conduits for the electrolytes. Depending on the physical nature of the foam, the substrates engender chemical activity, mass transport, electrical conductivity and fluid flow.

The making of porous metal foams falls into several categories. Some are made by melting the metal, blowing or creating gas bubbles in the melt and cooling it before the bubbles break and the gas escapes. Foams made this way are generally classified as closed cell foams. There are contiguous walls between each bubble of gas within the structure as in the case of soap lather. The gas in each bubble is discretely blocked off from the other bubbles. Such foams are useful for structural members since they possess the strength qualities of the metal phase without the full weight. They are used alone or in combination with other materials.

In contrast, open celled foams are those wherein a significant portion of each wall between the cells or bubbles has been destroyed leaving only struts or ligaments at the former intersections of the bubbles. These discontinuities result in windows between the cells creating a continuous path, in all directions, between the larger cells. Open celled foams tend to be used as frameworks or skeletons for holding other materials or as filters. The value of these structures in such applications is such that processes have been developed to modify traditional open cell metal foams by coating additional metal or ceramic onto the struts and ligaments of the metal foams to enhance the surface area prior to treating them with the desired material.

A variation of the open cell structure results when the metal forming the struts or ligaments (both terms may be used interchangeably) around the initial bubbles of gas is not derived from molten metal but from metal particles gently fused or sintered together. In this case, the struts rather than being closed and impervious are largely, in contrast, porous. The foam is comprised of less than 100% metal, sometimes much less, and extensive void space. U.S. Pat. No. 5,848,351 to Hoshino, et al., claims struts having porosity as high as 60%, i.e. a metal content of only 40%. Foams of this sort may be referred to as metal biporous foams since they possess both macro porosity resulting from the gas bubbles forming the joined cells and micro porosity resulting from the void space within the struts. The overall or bulk porosity of these foams is the average of the two levels of porosity in the foam. Therefore, altering either the micro or strut porosity or the macro or bubble porosity will change the bulk porosity.

These structures by virtue of their very high bulk porosity and high surface area may find applications as catalyst beds in fuel cells and other devices. The advantage of having porous struts is that the pores or voids in the struts may be intentionally filled with an agent designed to interact with another agent contained in a fluid. The fluid will pass easily through the large interconnected pores between the struts allowing the agents contained in the struts to react with those contained in the fluid as in the case of a catalyst bed.

In other applications, the small pores through the struts may simply trap contaminants contained in the fluid as they become lodged in the small pores without reducing the flow of the fluid through the body of the structure. Liquid contaminants in a gas would coalesce in and around the struts and drain away under gravity. In all of these cases, the porous struts play an integral role in the function of the structure.

Manufacturing foam by means of chemical reactions generating a gas results in a variation in the size of the bubbles or porosity or texture of the foam through its height due to the weight of material above the forming bubble. Mechanical and physical methods for producing foam are not burdened with this issue. However, these latter methods tend to be violent and will damage the delicate structure of the metal powders contemplated by present invention. Thus, generating the foam in a separate operation, followed by the addition of the powder (as taught by U.S. Pat. No. 4,569,821 to Duperray et al.) appeared to be promising. Unfortunately this method degrades the initial foam and cannot be used when using metal powders.

Other examples of metal biporous foams include:

U.S. Pat. No. 5,976,454 to Sterzel et al. discloses the use of dissolved gas, CO₂ or water (steam) to generate the foam but adds high temperature to speed evaporation to thicken the foam matrix to arrest the foaming process.

U.S. Pat. No. 5,848,351 to Hoshino et al. (above) discloses the use of volatile organic solvents that evaporate upon heating forming the foam. They sinter only partially and leave the micro porosity intact. The organics present a fire and environmental issue. Moreover, there is no control over bubble size.

U.S. Pat. No. 4,569,821 to Duperray et al. (above) discloses the use of a water-activated polymer to stabilize the foam after adding the metal powder on the foam. This process requires the use of a gelling agent to prevent the destruction of the foam when the metal powder is added. Adding the metal as dry powder draws water from the foam structure causing it to collapse. The original character of the foam is altered greatly by the addition of metal powder in this way. It also incorporates into the mixture a pocket of air surrounding each particle or agglomeration of particles which later contributes to the microstructure of the foam to an uncontrolled extent.

U.S. Pat. No. 5,213,612 to Minnear et al. discloses a method of forming a porous body of molybdenum, tungsten, and their respective alloys by mixing metal powder and a foaming agent dissolved in an organic solvent and sintered. There are fire and environmental issues caused by this process.

U.S. Pat. No. 6,087,024 to Whinnery et al. discloses a siloxane based foaming process. Volatization of the combined hydroxide functional siloxene and the hydride functional siloxane leads to environmental concerns.

U.S. Pat. No. 6,660,224 B2 to Lefebvre et al. discloses a foaming process that utilizes organic solvents.

There is needed a low cost environmentally friendly method for producing metal biporous foams, preferably using generally recognized as safe (“GRAS”) products and processes as much as possible.

SUMMARY OF THE INVENTION

There is provided a method for producing metal biporous foams by utilizing a solution of filamentary metal powders. A thickened cellulose based foam precursor and the wet metal powder mixture solution are mixed together. Foaming of the precursor is caused to occur. Once completed the resultant foam is moderately dried to form a green cake. The green cake is sintered in a reducing atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of an embodiment of the invention.

FIG. 2 is a photomicrograph of an embodiment of the invention.

FIG. 3 is a photomicrograph of an embodiment of the invention.

FIG. 4 is a photomicrograph of an embodiment of the invention.

FIG. 5 is a photomicrograph of an embodiment of the invention.

FIG. 6 is a photomicrograph of foam made in accordance with the invention attached to a copper pipe.

PREFERRED EMBODIMENT OF THE INVENTION

The present method is an environmentally friendly process for making high porosity metal biporous foam using GRAS materials or their derivatives (which in the latter case may not all be GRAS members).

Using traditional methods and traditional atomized metal particles, commercially available struts have a porosity of about 10%-60% or a metal content only as low as 40%. In contrast, the present process of using filamentary carbonyl derived metal powders results in struts having about 85%-95% porosity or a metal density of about 5%-15%.

The term “about” before a series of values, unless otherwise indicated, shall apply to each value in the series.

The term “filamentary” means, a characteristic three-dimensional chain-like network of fine or extra fine particles exhibited, for non-limiting example, by Inco® T255 nickel powder.

The assignee of the present invention (Inco Limited) produces and sells a series of ultra fine and exceedingly pure filamentary metal powders derived from the dissolution of metal carbonyl compounds via the exquisite Mond process.

Although the present process preferably uses such filamentary metal powders to provide the significantly enhanced product, metal powders produced by other methods may be employed to good advantage as well.

Foams can be generated by any of several methods as known to those skilled in the art. Some of the processes are described below:

1. Whipping air into a foamable solution by a mechanical means is one way of creating foam. The sizes of the bubbles in the foam will reflect the lifetime of the bubble since recently formed bubbles will not have had the opportunity to be comminuted to smaller bubbles. Thus such a method will result in bubbles of varying sizes.

2. Sudden release of the pressure on a dissolved gas by directing the mixture through a throttling device will also create foam. Once the solution surrounding each bubble has been depleted of the dissolved gas by diffusion into the bubble the bubbles stop growing. In this case, the bubbles will be much closer in size since they will have essentially the same lifetime.

3. Foam can also be created continuously by mechanical means such as those used in the fire fighting and foam insulation fields.

The preferred use of methylcellulose (MC) or hydroxypropyl methylcellulose (HPMC) as the major foam starter component of foam only requires raising the temperature to the thermal gelation temperature of the aqueous solution to solidify the structure. It is not necessary to completely drive off the water for this purpose. The remaining water is removed during sintering. The gelled structure does need to be at least partially dried to prevent it from returning to its slurry state upon cooling.

Once the stable foam has been created, the nickel powder is combined with it. As described in U.S. Pat. No. 4,569,821 above, adding dry metal powder degrades the foam. This debilitating issue is addressed and overcome by the present invention in the following manner. The nickel powder is first wetted with a solution of water and a wetting agent, for example a surfactant such as household dish washing liquid, to displace the air from around the particles or agglomerates of particles before mixing with the foam. The wet nickel solution mixture and the foam starter are added together with gentle mixing in order not to degrade the structure of either the foam or the nickel. The character of the final product can be controlled at this point by controlling the density of the foam while the nickel additions are combined with it. A foam density of about 0.5 g/cc results in a good product.

The wet foam is then dried or baked to stabilize or harden the structure while the water is driven out. In the case of MC, the foam will stabilize once it has been heated to above the thermal gelation temperature. Since the foam is initially a closed cell structure, changes in temperature will result in changes in the size of the bubbles. As water is driven out of the foam, the cell walls dry and the structure changes from closed to open cell allowing the liberation of the entrained gas and the free evaporation of the entrained water. Thus, the foam will stretch beyond its stabilized dimension while in the closed cell state but revert to its stabilized dimension after later changing to an open cell. The result is a dried green cake.

Sintering the green cake at high temperature in an appropriate atmosphere results in a metal foam monolith. Some methods sinter the green cake so severely that the metal particles melt together forming smooth filled struts. In contrast, the present method does not require a severe sinter and therefore results in the desirable porous struts.

Since the present method preferably uses filamentary metal powder, the porosity of the struts is normally at least about 80% rather than the lower 60% porosity when using non-filamentary powders, and less than about 20% nickel in the struts as metal compared with 40% for the prior art such as U.S. Pat. No. 5,868,351 as above. Indeed, the present process results in an overall porosity that is very high, up to about 95% or less than about 5% nickel.

Such a high porosity metal biporous foam has a great ability to filter solids from fluids and liquids from gases. In addition, the metal structure can be made magnetic (if appropriate) by the application of a magnetic field and thus filters metal cuttings and filings from cutting fluids and releases them, when cleaning the filter, upon removal of the magnetic field.

The present method preferably uses the following ingredients:

A. Binder, methylcellulose (“MC”) from the GRAS group or its derivatives, which may not all be GRAS.

B. Foaming agent including air, carbon dioxide, or nitrous oxide depending on the method of generating the foam.

C. Surface-active agents (surfactants) such as household dishwashing detergent.

D. Other benign agents such as glycerin, readily accessible to the general public and also GRAS.

More particularly, the following ingredients are preferably employed in the following non-limiting example:

3 g of MC (or equivalent types that give 4000 cp viscosity in a 2% solution)

-   -   100 g 0.5% dish washing solution (“DWS” typically SUNLIGHT® dish         washing liquid in water)     -   50 g INCO® Type 255 nickel powder     -   0.8g glycerin     -   25 g hot water (>70° C.)

1. Add 32 g of the DWS to the nickel powder and gently mix to completely wet the nickel powder. This solution mixture is then set aside to be used in step 8 below.

2. To vigorously stirred hot water in a vessel, slowly add the MC powder. Stirring and heating continue for 5 minutes.

3. Remove the MC from heat and, with stirring, slowly add the remaining DWS. By the time the DWS has been completely added, the MC solution has begun to thicken to form a foam precursor.

4. Optionally transfer the MC solution foam precursor from the original vessel to a larger one that will allow the subsequent beating or whipping process.

5. Allow the MC solution foam precursor sufficient time about, 30 minutes in this example, to thicken and become a sticky paste. Different types of MC require different conditions to affect this condition. Follow the instructions of the MC manufacturer.

6. After the MC foam precursor has thickened, add the glycerin to the foam precursor and mix gently to avoid making foam at this point. The glycerin promotes the longevity of the foam.

7. Using a mixer, such as a kitchen mixer that would be used for mixing cakes and the like, beat air into the MC solution foam precursor to create foam. Periodically during the process, remove a sample of the foam and weigh it to determine the density of the foam.

8. When the foam density is reduced to the desired target value, slowly add the foam, with gentle mixing, to the wet nickel powder solution mixture from step 1. Because the powder is already wet, it does not extract water from the foam structure as dry powder would and therefore does not damage it in any significant way. When the foam has all been added to the nickel powder and the mixture completely gently mixed, it is preferred to again sample the foam to determine its density. It has been determined that a foam density of about 0.5 g/cc gives a good product.

9. The foam is transferred to a mold or pan for drying.

10. The wet foam is dried in a humid oven at 250° F. (121° C.) for two hours. A hotter oven results in much expansion of the air bubbles in the foam causing the foam to collapse after about 30 minutes into the drying process.

11. After the foam has been dried, the resulting “green cake”, is sintered in a furnace under wet nitrogen and 10% hydrogen at 850° C. for one hour.

The resulting nickel biporous foam is mechanically robust and has a porosity of about 95% or a density of about 5% nickel, distributed throughout in a biporous structure.

In this process, the variation in macro pore size of the final product is determined by the uniformity of the original foam and therefore, any other suitable method for making the foam can be applied in order to achieve the desired texture in the final product.

The nickel biporous foam can be shaped at various stages in the process including wet foam, green foam or the final sintered foam.

FIG. 1 is a sintered metal biporous foam as produced in accordance with the above example. Scales along the putative x and y axis provide a physical sense of the product.

FIGS. 2 and 3 demonstrate the macroporosity of the foam at two selected magnifications.

FIG. 4 demonstrates the microporosity of a strut.

FIG. 5 demonstrates the microporosity of the foam at high magnification.

Thus, it can be seen that the present process results in an extremely porous metal biporous foam of desirable characteristics using relatively benign ingredients with little or no adverse impact to environment.

It should be apparent to one skilled in the art that commercial modifications will be made to the above example to ramp it up for industrial uses. Nonetheless, the principles will essentially remain the same albeit on a larger scale.

In order to test the efficacy of the present metal foam, the wet metal biporous foam of the present invention was applied to the interior surface of a short length of copper pipe and then dried.

After drying, the pipe and metal biporous foam were heated at 950° C. for ten minutes. The foam adhered tightly to the copper surface. FIG. 6 is a scanning electron microscope (“SEM”) microphotograph that reveals that the copper diffused into the nickel binding them permanently together. This allows an alloy foam to form. The diffusion of the metals effectively welds or brazes the materials together generating an extremely strong bond.

The table below tracks the location of the concentration percentage of the metals in the commingled metal foam of FIG. 6. Concentration of metals Location % Ni % Cu A 0.74 99.3 B 5.30 94.7 C 35.1 64.9 D 71.1 28.9

By varying the quantity of gas incorporated into the foam, the metal slurry is made into foam with different foam densities. Moreover, different types of gases dissolved in the slurry such as air, carbon dioxide, nitrogen, nitrous oxide, etc. may result in different foam textures and other physical and chemical properties since they affect the foam. Similarly, by altering the foam starter, MC, MC derivatives, molecular weights, GRAS binders, starch, surfactants and other concentrations the size of the bubbles and the foam's composition may be modified.

The invention is not limited to only one metal. Other metal powders such as copper, iron, nickel-based alloys, copper-base alloys, iron-base alloys etc. may be utilized singly instead or be mixed with the nickel powder or other metal powders that preferably display similar filamentary structures to those of the nickel particles.

The present process easily lends itself to the formation of multimetal and alloy metal biporous foams.

While in accordance with the provisions of the statute, there is illustrated and described herein specific embodiments of the invention. Those skilled in the art will understand that changes may be made in the form of the invention covered by the claims and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features. 

1. A process for producing metal biporous foam comprising; a) providing metal powder, b) wetting and mixing the metal powder with a liquid to form a wet metal containing solution mixture, c) creating a foam from a foam precursor, d) combining the foam and the wet metal containing solution mixture together and mixing same, e) drying the foam to form a green cake, and f) sintering the green cake to form a metal biporous foam.
 2. The process according to claim 1 wherein the metal powder is filamentary powder.
 3. The process according to claim 1 wherein the metal powder is selected from at least one of the group consisting of nickel, copper, iron, nickel-base alloys, copper-base alloys, and iron-base alloys.
 4. The process according to claim 1 including forming the foam from at least one of the group consisting of methylcellulose and hydroxylpropyl methylcellulose.
 5. The process according to claim 1 including mixing the metal powder with a surfactant to form the wet metal containing solution mixture.
 6. The process according to claim 1 including adding water to the metal powder to form the wet metal containing solution mixture.
 7. The process according to claim 1 including creating the foam by adding water to the foam former to form a foam precursor and allowing the foam precursor to thicken.
 8. The process according to claim 7 wherein glycerin is added to the foam precursor.
 9. The process according to claim 1 including forming the foam to a density of about 0.5 g/cc.
 10. The process according to claim 1 wherein the metal biporous foam has a porosity of about 85%-95%.
 11. The process according to claim 1 wherein the metal biporous foam has a density of about 5%-15% metal.
 12. The process according to claim 2 wherein the filamentary powder is derived from a metal carbonyl source.
 13. The process according to claim 1 wherein a foaming agent selected from at least one of the group consisting of air, carbon dioxide and nitrous dioxide creates the foam.
 14. The process according to claim 1 wherein mechanical means create the foam.
 15. The process according to claim 1 wherein the foam is heated to above its thermal gelation temperature.
 16. A process for producing metal biporous foam comprising: a) providing metal powder, b) wetting and mixing the metal powder with a liquid to form a wet metal mixture, c) providing a foam, d) combining the foam and the wet metal mixture, e) drying the combined foam and wet metal mixture to form a green cake, and f) sintering the green cake to form a metal biporous foam.
 17. The process according to claim 16 wherein the metal powder is filamentary powder.
 18. The process according to claim 16 wherein the metal biporous foam has a porosity of about 85%-95%.
 19. The process according to claim 16 wherein the metal biporous foam has a density of about 5%-15% metal.
 20. The process according to claim 16 wherein the metal powder is derived from a carbonyl source.
 21. The process according to claim 16 wherein the combined foam and wet metal mixture is heated to above its thermal gelation temperature. 